Ultraviolet matrix assisted laser desorption ionization-mass spectrometry of electroblotted proteins

Direct mass spectrometric analysis of proteins electroblotted onto polyvinylidene fluoride membranes after sodium dodecyl sulfate-polyacrylamide gel electrophoresis is demonstrated by matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS) with a linear time-of-flight instrument, equipped with a nitrogen laser (337 nm). The blotted proteins were desorbed directly from the blotting membrane after incubation with sinapinic acid as matrix. Different commercially available membranes resulted in high quality protein signals for hydrophobic membranes exhibiting high specific surface areas (Immobilon PSQ or Trans-Blot) or for charged membranes (Immobilon CD). Systematic investigations with standard proteins were performed to compare standard preparation procedures for ultraviolet (UV) MALDI-MS on stainless steel sample stages and preparation of proteins immobilized onto membranes either by direct application from protein solutions (spotting) or by electrotransfer from gels (electroblotting). Aspects such as mass resolution reproducibility from shot to shot and spot to spot, mass accuracy, and preservation of protein localization are addressed in this paper.